-
Notifications
You must be signed in to change notification settings - Fork 11
/
Copy pathOverview.html
4788 lines (4620 loc) · 344 KB
/
Overview.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!doctype html><html lang="en">
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
<title>Incremental Font Transfer</title>
<meta content="ED" name="w3c-status">
<link href="https://www.w3.org/StyleSheets/TR/2021/W3C-ED" rel="stylesheet">
<meta content="Bikeshed version d765c696b, updated Fri Mar 8 15:58:52 2024 -0800" name="generator">
<link href="https://www.w3.org/TR/IFT/" rel="canonical">
<meta content="69094a202d6402b4968a6d667b25e3fad4dcd3af" name="revision">
<meta content="dark light" name="color-scheme">
<link href="https://www.w3.org/StyleSheets/TR/2021/dark.css" media="(prefers-color-scheme: dark)" rel="stylesheet" type="text/css">
<style>
.conform:hover {background: #31668f; color: white}
.conform:target {padding: 2px; border: 2px solid #AAA; background: #31668f; color: white }
table {
width: 100%;
}
table, tr {
border: 1px solid #aaa;
border-collapse: collapse;
}
th, td {
padding: 0.5rem;
}
</style>
<style>/* Boilerplate: style-autolinks */
.css.css, .property.property, .descriptor.descriptor {
color: var(--a-normal-text);
font-size: inherit;
font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
content: "‘";
}
.css::after, .property::after, .descriptor::after {
content: "’";
}
.property, .descriptor {
/* Don't wrap property and descriptor names */
white-space: nowrap;
}
.type { /* CSS value <type> */
font-style: italic;
}
pre .property::before, pre .property::after {
content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
content: "’";
}
[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
content: "";
}
[data-link-type=element],
[data-link-type=element-attr] {
font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after { content: ">" }
[data-link-type=biblio] {
white-space: pre;
}
@media (prefers-color-scheme: dark) {
:root {
--selflink-text: black;
--selflink-bg: silver;
--selflink-hover-text: white;
}
}
</style>
<style>/* Boilerplate: style-colors */
/* Any --*-text not paired with a --*-bg is assumed to have a transparent bg */
:root {
color-scheme: light dark;
--text: black;
--bg: white;
--unofficial-watermark: url(https://www.w3.org/StyleSheets/TR/2016/logos/UD-watermark);
--logo-bg: #1a5e9a;
--logo-active-bg: #c00;
--logo-text: white;
--tocnav-normal-text: #707070;
--tocnav-normal-bg: var(--bg);
--tocnav-hover-text: var(--tocnav-normal-text);
--tocnav-hover-bg: #f8f8f8;
--tocnav-active-text: #c00;
--tocnav-active-bg: var(--tocnav-normal-bg);
--tocsidebar-text: var(--text);
--tocsidebar-bg: #f7f8f9;
--tocsidebar-shadow: rgba(0,0,0,.1);
--tocsidebar-heading-text: hsla(203,20%,40%,.7);
--toclink-text: var(--text);
--toclink-underline: #3980b5;
--toclink-visited-text: var(--toclink-text);
--toclink-visited-underline: #054572;
--heading-text: #005a9c;
--hr-text: var(--text);
--algo-border: #def;
--del-text: red;
--del-bg: transparent;
--ins-text: #080;
--ins-bg: transparent;
--a-normal-text: #034575;
--a-normal-underline: #bbb;
--a-visited-text: var(--a-normal-text);
--a-visited-underline: #707070;
--a-hover-bg: rgba(75%, 75%, 75%, .25);
--a-active-text: #c00;
--a-active-underline: #c00;
--blockquote-border: silver;
--blockquote-bg: transparent;
--blockquote-text: currentcolor;
--issue-border: #e05252;
--issue-bg: #fbe9e9;
--issue-text: var(--text);
--issueheading-text: #831616;
--example-border: #e0cb52;
--example-bg: #fcfaee;
--example-text: var(--text);
--exampleheading-text: #574b0f;
--note-border: #52e052;
--note-bg: #e9fbe9;
--note-text: var(--text);
--noteheading-text: hsl(120, 70%, 30%);
--notesummary-underline: silver;
--assertion-border: #aaa;
--assertion-bg: #eee;
--assertion-text: black;
--advisement-border: orange;
--advisement-bg: #fec;
--advisement-text: var(--text);
--advisementheading-text: #b35f00;
--warning-border: red;
--warning-bg: hsla(40,100%,50%,0.95);
--warning-text: var(--text);
--amendment-border: #330099;
--amendment-bg: #F5F0FF;
--amendment-text: var(--text);
--amendmentheading-text: #220066;
--def-border: #8ccbf2;
--def-bg: #def;
--def-text: var(--text);
--defrow-border: #bbd7e9;
--datacell-border: silver;
--indexinfo-text: #707070;
--indextable-hover-text: black;
--indextable-hover-bg: #f7f8f9;
--outdatedspec-bg: rgba(0, 0, 0, .5);
--outdatedspec-text: black;
--outdated-bg: maroon;
--outdated-text: white;
--outdated-shadow: red;
--editedrec-bg: darkorange;
}
@media (prefers-color-scheme: dark) {
:root {
--text: #ddd;
--bg: black;
--unofficial-watermark: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='400' height='400'%3E%3Cg fill='%23100808' transform='translate(200 200) rotate(-45) translate(-200 -200)' stroke='%23100808' stroke-width='3'%3E%3Ctext x='50%25' y='220' style='font: bold 70px sans-serif; text-anchor: middle; letter-spacing: 6px;'%3EUNOFFICIAL%3C/text%3E%3Ctext x='50%25' y='305' style='font: bold 70px sans-serif; text-anchor: middle; letter-spacing: 6px;'%3EDRAFT%3C/text%3E%3C/g%3E%3C/svg%3E");
--logo-bg: #1a5e9a;
--logo-active-bg: #c00;
--logo-text: white;
--tocnav-normal-text: #999;
--tocnav-normal-bg: var(--bg);
--tocnav-hover-text: var(--tocnav-normal-text);
--tocnav-hover-bg: #080808;
--tocnav-active-text: #f44;
--tocnav-active-bg: var(--tocnav-normal-bg);
--tocsidebar-text: var(--text);
--tocsidebar-bg: #080808;
--tocsidebar-shadow: rgba(255,255,255,.1);
--tocsidebar-heading-text: hsla(203,20%,40%,.7);
--toclink-text: var(--text);
--toclink-underline: #6af;
--toclink-visited-text: var(--toclink-text);
--toclink-visited-underline: #054572;
--heading-text: #8af;
--hr-text: var(--text);
--algo-border: #456;
--del-text: #f44;
--del-bg: transparent;
--ins-text: #4a4;
--ins-bg: transparent;
--a-normal-text: #6af;
--a-normal-underline: #555;
--a-visited-text: var(--a-normal-text);
--a-visited-underline: var(--a-normal-underline);
--a-hover-bg: rgba(25%, 25%, 25%, .2);
--a-active-text: #f44;
--a-active-underline: var(--a-active-text);
--borderedblock-bg: rgba(255, 255, 255, .05);
--blockquote-border: silver;
--blockquote-bg: var(--borderedblock-bg);
--blockquote-text: currentcolor;
--issue-border: #e05252;
--issue-bg: var(--borderedblock-bg);
--issue-text: var(--text);
--issueheading-text: hsl(0deg, 70%, 70%);
--example-border: hsl(50deg, 90%, 60%);
--example-bg: var(--borderedblock-bg);
--example-text: var(--text);
--exampleheading-text: hsl(50deg, 70%, 70%);
--note-border: hsl(120deg, 100%, 35%);
--note-bg: var(--borderedblock-bg);
--note-text: var(--text);
--noteheading-text: hsl(120, 70%, 70%);
--notesummary-underline: silver;
--assertion-border: #444;
--assertion-bg: var(--borderedblock-bg);
--assertion-text: var(--text);
--advisement-border: orange;
--advisement-bg: #222218;
--advisement-text: var(--text);
--advisementheading-text: #f84;
--warning-border: red;
--warning-bg: hsla(40,100%,20%,0.95);
--warning-text: var(--text);
--amendment-border: #330099;
--amendment-bg: #080010;
--amendment-text: var(--text);
--amendmentheading-text: #cc00ff;
--def-border: #8ccbf2;
--def-bg: #080818;
--def-text: var(--text);
--defrow-border: #136;
--datacell-border: silver;
--indexinfo-text: #aaa;
--indextable-hover-text: var(--text);
--indextable-hover-bg: #181818;
--outdatedspec-bg: rgba(255, 255, 255, .5);
--outdatedspec-text: black;
--outdated-bg: maroon;
--outdated-text: white;
--outdated-shadow: red;
--editedrec-bg: darkorange;
}
/* In case a transparent-bg image doesn't expect to be on a dark bg,
which is quite common in practice... */
img { background: white; }
}
</style>
<style>/* Boilerplate: style-counters */
body {
counter-reset: example figure issue;
}
.issue {
counter-increment: issue;
}
.issue:not(.no-marker)::before {
content: "Issue " counter(issue);
}
.example {
counter-increment: example;
}
.example:not(.no-marker)::before {
content: "Example " counter(example);
}
.invalid.example:not(.no-marker)::before,
.illegal.example:not(.no-marker)::before {
content: "Invalid Example" counter(example);
}
figcaption {
counter-increment: figure;
}
figcaption:not(.no-marker)::before {
content: "Figure " counter(figure) " ";
}
</style>
<style>/* Boilerplate: style-dfn-panel */
:root {
--dfnpanel-bg: #ddd;
--dfnpanel-text: var(--text);
--dfnpanel-target-bg: #ffc;
--dfnpanel-target-outline: orange;
}
@media (prefers-color-scheme: dark) {
:root {
--dfnpanel-bg: #222;
--dfnpanel-text: var(--text);
--dfnpanel-target-bg: #333;
--dfnpanel-target-outline: silver;
}
}
.dfn-panel {
position: absolute;
z-index: 35;
width: 20em;
width: 300px;
height: auto;
max-height: 500px;
overflow: auto;
padding: 0.5em 0.75em;
font: small Helvetica Neue, sans-serif, Droid Sans Fallback;
background: var(--dfnpanel-bg);
color: var(--dfnpanel-text);
border: outset 0.2em;
white-space: normal; /* in case it's moved into a pre */
}
.dfn-panel:not(.on) { display: none; }
.dfn-panel * { margin: 0; padding: 0; text-indent: 0; }
.dfn-panel > b { display: block; }
.dfn-panel a { color: var(--dfnpanel-text); }
.dfn-panel a:not(:hover) { text-decoration: none !important; border-bottom: none !important; }
.dfn-panel a:focus {
outline: 5px auto Highlight;
outline: 5px auto -webkit-focus-ring-color;
}
.dfn-panel > b + b { margin-top: 0.25em; }
.dfn-panel ul { padding: 0 0 0 1em; list-style: none; }
.dfn-panel li a {
max-width: calc(300px - 1.5em - 1em);
overflow: hidden;
text-overflow: ellipsis;
}
.dfn-panel.activated {
display: inline-block;
position: fixed;
left: 8px;
bottom: 2em;
margin: 0 auto;
max-width: calc(100vw - 1.5em - .4em - .5em);
max-height: 30vh;
transition: left 1s ease-out, bottom 1s ease-out;
}
.dfn-panel .link-item:hover {
text-decoration: underline;
}
.dfn-panel .link-item .copy-icon {
opacity: 0;
}
.dfn-panel .link-item:hover .copy-icon,
.dfn-panel .link-item .copy-icon:focus {
opacity: 1;
}
.dfn-panel .copy-icon {
display: inline-block;
margin-right: 0.5em;
width: 0.85em;
height: 1em;
border-radius: 3px;
background-color: #ccc;
cursor: pointer;
}
.dfn-panel .copy-icon .icon {
width: 100%;
height: 100%;
background-color: #fff;
display: flex;
justify-content: center;
align-items: center;
position: relative;
}
.dfn-panel .copy-icon .icon::before {
content: "";
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
border: 1px solid black;
background-color: #ccc;
opacity: 0.25;
transform: translate(3px, -3px);
}
.dfn-panel .copy-icon:active .icon::before {
opacity: 1;
}
.dfn-paneled[role="button"] { cursor: help; }
.highlighted {
animation: target-fade 3s;
}
@keyframes target-fade {
from {
background-color: var(--dfnpanel-target-bg);
outline: 5px solid var(--dfnpanel-target-outline);
}
to {
color: var(--a-normal-text);
background-color: transparent;
outline: transparent;
}
}
</style>
<style>/* Boilerplate: style-issues */
a[href].issue-return {
float: right;
float: inline-end;
color: var(--issueheading-text);
font-weight: bold;
text-decoration: none;
}
</style>
<style>/* Boilerplate: style-md-lists */
/* This is a weird hack for me not yet following the commonmark spec
regarding paragraph and lists. */
[data-md] > :first-child {
margin-top: 0;
}
[data-md] > :last-child {
margin-bottom: 0;
}
</style>
<style>/* Boilerplate: style-ref-hints */
:root {
--ref-hint-bg: #ddd;
--ref-hint-text: var(--text);
}
@media (prefers-color-scheme: dark) {
:root {
--ref-hint-bg: #222;
--ref-hint-text: var(--text);
}
}
.ref-hint {
display: inline-block;
position: absolute;
z-index: 35;
width: 20em;
width: 300px;
height: auto;
max-height: 500px;
overflow: auto;
padding: 0.5em 0.5em;
font: small Helvetica Neue, sans-serif, Droid Sans Fallback;
background: var(--ref-hint-bg);
color: var(--ref-hint-text);
border: outset 0.2em;
white-space: normal; /* in case it's moved into a pre */
}
.ref-hint * { margin: 0; padding: 0; text-indent: 0; }
.ref-hint ul { padding: 0 0 0 1em; list-style: none; }
</style>
<style>/* Boilerplate: style-selflinks */
:root {
--selflink-text: white;
--selflink-bg: gray;
--selflink-hover-text: black;
}
.heading, .issue, .note, .example, li, dt {
position: relative;
}
a.self-link {
position: absolute;
top: 0;
left: calc(-1 * (3.5rem - 26px));
width: calc(3.5rem - 26px);
height: 2em;
text-align: center;
border: none;
transition: opacity .2s;
opacity: .5;
}
a.self-link:hover {
opacity: 1;
}
.heading > a.self-link {
font-size: 83%;
}
.example > a.self-link,
.note > a.self-link,
.issue > a.self-link {
/* These blocks are overflow:auto, so positioning outside
doesn't work. */
left: auto;
right: 0;
}
li > a.self-link {
left: calc(-1 * (3.5rem - 26px) - 2em);
}
dfn > a.self-link {
top: auto;
left: auto;
opacity: 0;
width: 1.5em;
height: 1.5em;
background: var(--selflink-bg);
color: var(--selflink-text);
font-style: normal;
transition: opacity .2s, background-color .2s, color .2s;
}
dfn:hover > a.self-link {
opacity: 1;
}
dfn > a.self-link:hover {
color: var(--selflink-hover-text);
}
a.self-link::before { content: "¶"; }
.heading > a.self-link::before { content: "§"; }
dfn > a.self-link::before { content: "#"; }
</style>
<style>/* Boilerplate: style-syntax-highlighting */
code.highlight { padding: .1em; border-radius: .3em; }
pre.highlight, pre > code.highlight { display: block; padding: 1em; margin: .5em 0; overflow: auto; border-radius: 0; }
.highlight:not(.idl) { background: rgba(0, 0, 0, .03); }
c-[a] { color: #990055 } /* Keyword.Declaration */
c-[b] { color: #990055 } /* Keyword.Type */
c-[c] { color: #708090 } /* Comment */
c-[d] { color: #708090 } /* Comment.Multiline */
c-[e] { color: #0077aa } /* Name.Attribute */
c-[f] { color: #669900 } /* Name.Tag */
c-[g] { color: #222222 } /* Name.Variable */
c-[k] { color: #990055 } /* Keyword */
c-[l] { color: #000000 } /* Literal */
c-[m] { color: #000000 } /* Literal.Number */
c-[n] { color: #0077aa } /* Name */
c-[o] { color: #999999 } /* Operator */
c-[p] { color: #999999 } /* Punctuation */
c-[s] { color: #a67f59 } /* Literal.String */
c-[t] { color: #a67f59 } /* Literal.String.Single */
c-[u] { color: #a67f59 } /* Literal.String.Double */
c-[cp] { color: #708090 } /* Comment.Preproc */
c-[c1] { color: #708090 } /* Comment.Single */
c-[cs] { color: #708090 } /* Comment.Special */
c-[kc] { color: #990055 } /* Keyword.Constant */
c-[kn] { color: #990055 } /* Keyword.Namespace */
c-[kp] { color: #990055 } /* Keyword.Pseudo */
c-[kr] { color: #990055 } /* Keyword.Reserved */
c-[ld] { color: #000000 } /* Literal.Date */
c-[nc] { color: #0077aa } /* Name.Class */
c-[no] { color: #0077aa } /* Name.Constant */
c-[nd] { color: #0077aa } /* Name.Decorator */
c-[ni] { color: #0077aa } /* Name.Entity */
c-[ne] { color: #0077aa } /* Name.Exception */
c-[nf] { color: #0077aa } /* Name.Function */
c-[nl] { color: #0077aa } /* Name.Label */
c-[nn] { color: #0077aa } /* Name.Namespace */
c-[py] { color: #0077aa } /* Name.Property */
c-[ow] { color: #999999 } /* Operator.Word */
c-[mb] { color: #000000 } /* Literal.Number.Bin */
c-[mf] { color: #000000 } /* Literal.Number.Float */
c-[mh] { color: #000000 } /* Literal.Number.Hex */
c-[mi] { color: #000000 } /* Literal.Number.Integer */
c-[mo] { color: #000000 } /* Literal.Number.Oct */
c-[sb] { color: #a67f59 } /* Literal.String.Backtick */
c-[sc] { color: #a67f59 } /* Literal.String.Char */
c-[sd] { color: #a67f59 } /* Literal.String.Doc */
c-[se] { color: #a67f59 } /* Literal.String.Escape */
c-[sh] { color: #a67f59 } /* Literal.String.Heredoc */
c-[si] { color: #a67f59 } /* Literal.String.Interpol */
c-[sx] { color: #a67f59 } /* Literal.String.Other */
c-[sr] { color: #a67f59 } /* Literal.String.Regex */
c-[ss] { color: #a67f59 } /* Literal.String.Symbol */
c-[vc] { color: #0077aa } /* Name.Variable.Class */
c-[vg] { color: #0077aa } /* Name.Variable.Global */
c-[vi] { color: #0077aa } /* Name.Variable.Instance */
c-[il] { color: #000000 } /* Literal.Number.Integer.Long */
@media (prefers-color-scheme: dark) {
.highlight:not(.idl) { background: rgba(255, 255, 255, .05); }
c-[a] { color: #d33682 } /* Keyword.Declaration */
c-[b] { color: #d33682 } /* Keyword.Type */
c-[c] { color: #2aa198 } /* Comment */
c-[d] { color: #2aa198 } /* Comment.Multiline */
c-[e] { color: #268bd2 } /* Name.Attribute */
c-[f] { color: #b58900 } /* Name.Tag */
c-[g] { color: #cb4b16 } /* Name.Variable */
c-[k] { color: #d33682 } /* Keyword */
c-[l] { color: #657b83 } /* Literal */
c-[m] { color: #657b83 } /* Literal.Number */
c-[n] { color: #268bd2 } /* Name */
c-[o] { color: #657b83 } /* Operator */
c-[p] { color: #657b83 } /* Punctuation */
c-[s] { color: #6c71c4 } /* Literal.String */
c-[t] { color: #6c71c4 } /* Literal.String.Single */
c-[u] { color: #6c71c4 } /* Literal.String.Double */
c-[ch] { color: #2aa198 } /* Comment.Hashbang */
c-[cp] { color: #2aa198 } /* Comment.Preproc */
c-[cpf] { color: #2aa198 } /* Comment.PreprocFile */
c-[c1] { color: #2aa198 } /* Comment.Single */
c-[cs] { color: #2aa198 } /* Comment.Special */
c-[kc] { color: #d33682 } /* Keyword.Constant */
c-[kn] { color: #d33682 } /* Keyword.Namespace */
c-[kp] { color: #d33682 } /* Keyword.Pseudo */
c-[kr] { color: #d33682 } /* Keyword.Reserved */
c-[ld] { color: #657b83 } /* Literal.Date */
c-[nc] { color: #268bd2 } /* Name.Class */
c-[no] { color: #268bd2 } /* Name.Constant */
c-[nd] { color: #268bd2 } /* Name.Decorator */
c-[ni] { color: #268bd2 } /* Name.Entity */
c-[ne] { color: #268bd2 } /* Name.Exception */
c-[nf] { color: #268bd2 } /* Name.Function */
c-[nl] { color: #268bd2 } /* Name.Label */
c-[nn] { color: #268bd2 } /* Name.Namespace */
c-[py] { color: #268bd2 } /* Name.Property */
c-[ow] { color: #657b83 } /* Operator.Word */
c-[mb] { color: #657b83 } /* Literal.Number.Bin */
c-[mf] { color: #657b83 } /* Literal.Number.Float */
c-[mh] { color: #657b83 } /* Literal.Number.Hex */
c-[mi] { color: #657b83 } /* Literal.Number.Integer */
c-[mo] { color: #657b83 } /* Literal.Number.Oct */
c-[sa] { color: #6c71c4 } /* Literal.String.Affix */
c-[sb] { color: #6c71c4 } /* Literal.String.Backtick */
c-[sc] { color: #6c71c4 } /* Literal.String.Char */
c-[dl] { color: #6c71c4 } /* Literal.String.Delimiter */
c-[sd] { color: #6c71c4 } /* Literal.String.Doc */
c-[se] { color: #6c71c4 } /* Literal.String.Escape */
c-[sh] { color: #6c71c4 } /* Literal.String.Heredoc */
c-[si] { color: #6c71c4 } /* Literal.String.Interpol */
c-[sx] { color: #6c71c4 } /* Literal.String.Other */
c-[sr] { color: #6c71c4 } /* Literal.String.Regex */
c-[ss] { color: #6c71c4 } /* Literal.String.Symbol */
c-[fm] { color: #268bd2 } /* Name.Function.Magic */
c-[vc] { color: #cb4b16 } /* Name.Variable.Class */
c-[vg] { color: #cb4b16 } /* Name.Variable.Global */
c-[vi] { color: #cb4b16 } /* Name.Variable.Instance */
c-[vm] { color: #cb4b16 } /* Name.Variable.Magic */
c-[il] { color: #657b83 } /* Literal.Number.Integer.Long */
}
</style>
<style>/* Boilerplate: style-var-click-highlighting */
/*
Colors were chosen in Lab using https://nixsensor.com/free-color-converter/
D50 2deg illuminant, L in [0,100], a and b in [-128, 128]
0 = lab(85,0,85)
1 = lab(85,80,30)
2 = lab(85,-40,40)
3 = lab(85,-50,0)
4 = lab(85,5,15)
5 = lab(85,-10,-50)
6 = lab(85,35,-15)
*/
[data-algorithm] var { cursor: pointer; }
var[data-var-color="0"] { background-color: #F4D200; box-shadow: 0 0 0 2px #F4D200; }
var[data-var-color="1"] { background-color: #FF87A2; box-shadow: 0 0 0 2px #FF87A2; }
var[data-var-color="2"] { background-color: #96E885; box-shadow: 0 0 0 2px #96E885; }
var[data-var-color="3"] { background-color: #3EEED2; box-shadow: 0 0 0 2px #3EEED2; }
var[data-var-color="4"] { background-color: #EACFB6; box-shadow: 0 0 0 2px #EACFB6; }
var[data-var-color="5"] { background-color: #82DDFF; box-shadow: 0 0 0 2px #82DDFF; }
var[data-var-color="6"] { background-color: #FFBCF2; box-shadow: 0 0 0 2px #FFBCF2; }
</style>
<body class="h-entry">
<div class="head">
<p data-fill-with="logo"><a class="logo" href="https://www.w3.org/"> <img alt="W3C" height="48" src="https://www.w3.org/StyleSheets/TR/2021/logos/W3C" width="72"> </a> </p>
<h1 class="p-name no-ref" id="title">Incremental Font Transfer</h1>
<p id="w3c-state"><a href="https://www.w3.org/standards/types#ED">Editor’s Draft</a>, <time class="dt-updated" datetime="2025-01-31">31 January 2025</time></p>
<div data-fill-with="spec-metadata">
<dl>
<dt>This version:
<dd><a class="u-url" href="https://w3c.github.io/IFT/Overview.html">https://w3c.github.io/IFT/Overview.html</a>
<dt>Latest published version:
<dd><a href="https://www.w3.org/TR/IFT/">https://www.w3.org/TR/IFT/</a>
<dt>Feedback:
<dd><span><a href="mailto:[email protected]?subject=%5BIFT%5D%20YOUR%20TOPIC%20HERE">[email protected]</a> with subject line “<kbd>[IFT] <i data-lt>… message topic …</i></kbd>” (<a href="https://lists.w3.org/Archives/Public/public-webfonts-wg/" rel="discussion">archives</a>)</span>
<dd><a href="https://github.com/w3c/PFE/issues/">GitHub</a>
<dt class="editor">Editors:
<dd class="editor p-author h-card vcard" data-editor-id="1438"><a class="p-name fn u-url url" href="https://svgees.us/">Chris Lilley</a> (<span class="p-org org">W3C</span>)
<dd class="editor p-author h-card vcard" data-editor-id="73905"><a class="p-name fn u-email email" href="mailto:[email protected]">Garret Rieger</a> (<span class="p-org org">Google Inc.</span>)
<dd class="editor p-author h-card vcard" data-editor-id="137857"><a class="p-name fn u-email email" href="mailto:[email protected]">Skef Iterum</a> (<span class="p-org org">Adobe Inc.</span>)
</dl>
</div>
<div data-fill-with="warning"></div>
<p class="copyright" data-fill-with="copyright"><a href="https://www.w3.org/policies/#copyright">Copyright</a> © 2025 <a href="https://www.w3.org/">World Wide Web Consortium</a>. <abbr title="World Wide Web Consortium">W3C</abbr><sup>®</sup> <a href="https://www.w3.org/policies/#Legal_Disclaimer">liability</a>, <a href="https://www.w3.org/policies/#W3C_Trademarks">trademark</a> and <a href="https://www.w3.org/copyright/software-license/" rel="license" title="W3C Software and Document License">permissive document license</a> rules apply. </p>
<hr title="Separator for header">
</div>
<div class="p-summary" data-fill-with="abstract">
<h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
<p>This specification defines a method to incrementally transfer a font
from server to client. The client loads only the portion(s) of the font that
they actually need, significantly reducing data transfer. Multiple incremental
additions to the same font are possible, such as a user agent updating a font
as a user browses multiple pages. Incremental transfer improves on
unicode-range by avoiding damage to layout (kerning, ligatures, etc) rules,
meaning it can efficiently support fine grained increments to latin and
to complex scripts like Indic or Arabic.</p>
</div>
<h2 class="no-num no-toc no-ref heading settled" id="sotd"><span class="content">Status of this document</span></h2>
<div data-fill-with="status">
<p> This is a public copy of the editors’ draft.
It is provided for discussion only and may change at any moment.
Its publication here does not imply endorsement of its contents by W3C.
Don’t cite this document other than as work in progress. </p>
<p> If you wish to make comments regarding this document, please <a href="https://github.com/w3c/PFE/issues/new">file an issue on the specification repository</a>. </p>
<p> This document was produced by the <a href="https://www.w3.org/groups/wg/webfonts">Web Fonts Working Group</a> </p>
<p> This document was produced by a group operating under
the <a href="https://www.w3.org/policies/patent-policy/20200915/">W3C Patent Policy</a>.
W3C maintains a <a href="https://www.w3.org/groups/wg/webfonts/ipr" rel="disclosure">public list of any patent disclosures</a> made in connection with the deliverables of the group;
that page also includes instructions for disclosing a patent.
An individual who has actual knowledge of a patent which the individual believes contains <a href="https://www.w3.org/policies/patent-policy/20200915/#def-essential">Essential Claim(s)</a> must disclose the information in accordance with <a href="https://www.w3.org/policies/patent-policy/20200915/#sec-Disclosure">section 6 of the <abbr title="World Wide Web Consortium">W3C</abbr> Patent Policy</a>. </p>
<p> This document is governed by the <a href="https://www.w3.org/policies/process/20231103/" id="w3c_process_revision">03 November 2023 W3C Process Document</a>. </p>
<p></p>
</div>
<div data-fill-with="at-risk"></div>
<nav data-fill-with="table-of-contents" id="toc">
<h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
<ol class="toc" role="directory">
<li>
<a href="#intro"><span class="secno">1</span> <span class="content">Introduction</span></a>
<ol class="toc">
<li><a href="#evaluation-report"><span class="secno">1.1</span> <span class="content">Technical Motivation: Evaluation Report</span></a>
<li><a href="#overview"><span class="secno">1.2</span> <span class="content">Overview</span></a>
<li><a href="#making-incremental-fonts"><span class="secno">1.3</span> <span class="content">Creating an Incremental Font</span></a>
<li>
<a href="#performance-considerations"><span class="secno">1.4</span> <span class="content">Performance Considerations and the use of Incremental Font Transfer</span></a>
<ol class="toc">
<li><a href="#reduce-requests"><span class="secno">1.4.1</span> <span class="content">Reducing the Number of Network Requests</span></a>
</ol>
</ol>
<li>
<a href="#opt-in"><span class="secno">2</span> <span class="content">Opt-In Mechanism</span></a>
<ol class="toc">
<li><a href="#offline-usage"><span class="secno">2.1</span> <span class="content">Offline Usage</span></a>
</ol>
<li>
<a href="#definitions"><span class="secno">3</span> <span class="content">Definitions</span></a>
<ol class="toc">
<li><a href="#font-subset-dfn"><span class="secno">3.1</span> <span class="content">Font Subset</span></a>
<li><a href="#font-patch-definitions"><span class="secno">3.2</span> <span class="content">Font Patch</span></a>
<li><a href="#patch-map-dfn"><span class="secno">3.3</span> <span class="content">Patch Map</span></a>
<li><a href="#data-types"><span class="secno">3.4</span> <span class="content">Explanation of Data Types</span></a>
</ol>
<li>
<a href="#extending-font-subset"><span class="secno">4</span> <span class="content">Extending a Font Subset</span></a>
<ol class="toc">
<li><a href="#font-patch-invalidations"><span class="secno">4.1</span> <span class="content">Patch Invalidations</span></a>
<li><a href="#default-layout-features"><span class="secno">4.2</span> <span class="content">Default Layout Features</span></a>
<li><a href="#extend-font-subset"><span class="secno">4.3</span> <span class="content">Incremental Font Extension Algorithm</span></a>
<li><a href="#invalidating-patch-selection"><span class="secno">4.4</span> <span class="content">Selecting Invalidating Patches</span></a>
<li><a href="#target-subset-definitions"><span class="secno">4.5</span> <span class="content">Target Subset Definition</span></a>
<li><a href="#ift-font-coverage"><span class="secno">4.6</span> <span class="content">Determining what Content a Font can Render</span></a>
<li><a href="#fully-expanding-a-font"><span class="secno">4.7</span> <span class="content">Fully Expanding a Font</span></a>
<li><a href="#caching-incremental-fonts"><span class="secno">4.8</span> <span class="content">Caching Extended Incremental Fonts</span></a>
</ol>
<li>
<a href="#font-format-extensions"><span class="secno">5</span> <span class="content">Extensions to the Font Format</span></a>
<ol class="toc">
<li><a href="#ift-and-compression"><span class="secno">5.1</span> <span class="content">Incremental Font Transfer and Font Compression Formats</span></a>
<li>
<a href="#patch-map-table"><span class="secno">5.2</span> <span class="content">Patch Map Table</span></a>
<ol class="toc">
<li>
<a href="#patch-map-format-1"><span class="secno">5.2.1</span> <span class="content">Patch Map Table: Format 1</span></a>
<ol class="toc">
<li><a href="#interpreting-patch-map-format-1"><span class="secno">5.2.1.1</span> <span class="content">Interpreting Format 1</span></a>
<li><a href="#remove-entries-format-1"><span class="secno">5.2.1.2</span> <span class="content">Remove Entries from Format 1</span></a>
</ol>
<li>
<a href="#patch-map-format-2"><span class="secno">5.2.2</span> <span class="content">Patch Map Table: Format 2</span></a>
<ol class="toc">
<li><a href="#interpreting-patch-map-format-2"><span class="secno">5.2.2.1</span> <span class="content">Interpreting Format 2</span></a>
<li><a href="#remove-entries-format-2"><span class="secno">5.2.2.2</span> <span class="content">Remove Entries from Format 2</span></a>
<li><a href="#sparse-bit-set-decoding"><span class="secno">5.2.2.3</span> <span class="content">Sparse Bit Set</span></a>
</ol>
<li><a href="#uri-templates"><span class="secno">5.2.3</span> <span class="content">URI Templates</span></a>
</ol>
</ol>
<li>
<a href="#font-patch-formats"><span class="secno">6</span> <span class="content">Font Patch Formats</span></a>
<ol class="toc">
<li><a href="#font-patch-formats-summary"><span class="secno">6.1</span> <span class="content">Formats Summary</span></a>
<li>
<a href="#table-keyed"><span class="secno">6.2</span> <span class="content">Table Keyed</span></a>
<ol class="toc">
<li><a href="#apply-table-keyed"><span class="secno">6.2.1</span> <span class="content">Applying Table Keyed Patches</span></a>
</ol>
<li>
<a href="#glyph-keyed"><span class="secno">6.3</span> <span class="content">Glyph Keyed</span></a>
<ol class="toc">
<li><a href="#apply-glyph-keyed"><span class="secno">6.3.1</span> <span class="content">Applying Glyph Keyed Patches</span></a>
</ol>
</ol>
<li>
<a href="#encoding"><span class="secno">7</span> <span class="content">Encoding</span></a>
<ol class="toc">
<li><a href="#encoding-considerations"><span class="secno">7.1</span> <span class="content">Encoding Considerations</span></a>
</ol>
<li>
<a href="#priv"><span class="secno">8</span> <span class="content">Privacy Considerations</span></a>
<ol class="toc">
<li><a href="#content-inference-from-character-set"><span class="secno">8.1</span> <span class="content">Content inference from character set</span></a>
<li><a href="#per-origin"><span class="secno">8.2</span> <span class="content">Per-origin restriction avoids fingerprinting</span></a>
</ol>
<li><a href="#sec"><span class="secno">9</span> <span class="content">Security Considerations</span></a>
<li><a href="#changes"><span class="secno">10</span> <span class="content">Changes</span></a>
<li><a href="#feature-tag-list"><span class="secno"></span> <span class="content"> Appendix A: Default Feature Tags</span></a>
<li><a href="#extension-example"><span class="secno"></span> <span class="content"> Appendix B: Extension Algorithm Example Execution</span></a>
<li>
<a href="#w3c-conformance"><span class="secno"></span> <span class="content">Conformance</span></a>
<ol class="toc">
<li><a href="#w3c-conventions"><span class="secno"></span> <span class="content">Document conventions</span></a>
<li><a href="#w3c-conformant-algorithms"><span class="secno"></span> <span class="content">Conformant Algorithms</span></a>
</ol>
<li>
<a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
<ol class="toc">
<li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
</ol>
<li>
<a href="#references"><span class="secno"></span> <span class="content">References</span></a>
<ol class="toc">
<li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
<li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
</ol>
</ol>
</nav>
<main>
<h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#intro"></a></h2>
<p><em>This section is not normative.</em></p>
<p>Incremental Font Transfer (IFT) is a technology to improve the latency of remote fonts (or "web fonts") on
the web. Without this technology, a browser needs to download every last byte of a font before it can render
any characters using that font. IFT allows the browser to download only some of the bytes in the file, thereby
decreasing the perceived latency between the time when a browser realizes it needs a font and when the
necessary text can be rendered with that font. Unlike traditional font subsetting approaches Incremental Font Transfer
retains the encoding of layout rules between segments (<a href="https://www.w3.org/TR/PFE-evaluation/#fail-subset">Progressive Font Enrichment: Evaluation Report § fail-subset</a>).</p>
<p>The success of WebFonts is unevenly distributed. This specification allows WebFonts to be used where
slow networks, very large fonts, or complex subsetting requirements currently preclude their use. For
example, even using WOFF 2 <a data-link-type="biblio" href="#biblio-woff2" title="WOFF File Format 2.0">[WOFF2]</a>, fonts for CJK languages can be too large to be practical.</p>
<h3 class="heading settled" data-level="1.1" id="evaluation-report"><span class="secno">1.1. </span><span class="content">Technical Motivation: Evaluation Report</span><a class="self-link" href="#evaluation-report"></a></h3>
<p>See the Progressive Font Enrichment: Evaluation Report <a data-link-type="biblio" href="#biblio-pfe-report" title="Progressive Font Enrichment: Evaluation Report">[PFE-report]</a> for the investigation which led
to this specification.</p>
<h3 class="heading settled" data-level="1.2" id="overview"><span class="secno">1.2. </span><span class="content">Overview</span><a class="self-link" href="#overview"></a></h3>
<p>An <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="incremental-font">incremental font</dfn> is a regular <a data-link-type="biblio" href="#biblio-open-type" title="OpenType Specification">OpenType</a> font that is reformatted to include incremental functionality,
partly in virtue of two additional <a href="https://docs.microsoft.com/en-us/typography/opentype/spec/otff#table-directory">tables</a>. Using these new tables the font can be augmented
(eg. to cover more code points) by loading and applying patches to it.</p>
<p>The IFT technology has four main pieces:</p>
<ul>
<li data-md>
<p><a href="#extending-font-subset">§ 4 Extending a Font Subset</a>: provides the algorithm that is used by a client to select and apply patches.</p>
<li data-md>
<p><a href="#font-format-extensions">§ 5 Extensions to the Font Format</a>: defines the new tables which contain a list of patches that are available to be applied to a font.</p>
<li data-md>
<p><a href="#font-patch-formats">§ 6 Font Patch Formats</a>: defines three different types of patches that can be used. Two are "generic" binary patches, one is
specific to the font’s format for storing glyph data.</p>
<li data-md>
<p><a href="#encoding">§ 7 Encoding</a>: creates the font and associated patches that form an incremental font.</p>
</ul>
<p>At a high level an <a data-link-type="dfn" href="#incremental-font" id="ref-for-incremental-font">incremental font</a> is used like this:</p>
<ol>
<li data-md>
<p>The client downloads an initial font file, which contains some initial subset of data from the full version of the font along with <a href="#font-format-extensions">embedded data</a> describing the set of <a href="#font-patch-formats">patches</a> which can be used to extend
the font.</p>
<li data-md>
<p>Based on the content to be rendered, the client <a href="#extending-font-subset">selects, downloads, and applies</a> patches to extend
the font to cover additional characters, layout features, and/or variation space. This step is repeated each time there is new
content.</p>
</ol>
<h3 class="heading settled" data-level="1.3" id="making-incremental-fonts"><span class="secno">1.3. </span><span class="content">Creating an Incremental Font</span><a class="self-link" href="#making-incremental-fonts"></a></h3>
<p>It is expected that the most common way to produce an incremental font will be to convert an existing font to use the incremental
encoding defined in this specification. At a high level converting an existing font to be incremental will look like this:</p>
<ol>
<li data-md>
<p>Choose the content of the initial <a href="#font-subset-dfn">subset</a>, this will be included in the initial font file the client loads
and usually consists of any data from the original font that is expected to always be needed.</p>
<li data-md>
<p>Choose the patch type or types. Different patch types have different qualities, so different use cases can call for different
patch types, or in some cases a mix of two types.</p>
<li data-md>
<p>Choose a segmentation of the font. Individual segments will be added to the base subset by the client using patches. Choosing an
appropriate segmentation is one of the most important parts of producing an efficient encoding.</p>
<li data-md>
<p>Based on these choices, generate a set of patches, where each patch adds the data for a particular segment relative to either
the initial font file or a previous patch.</p>
<li data-md>
<p>Generate the initial font file including the initial subset and a <a href="#font-format-extensions">patch mapping</a>. This mapping
lists all of the available patch files, the url they reside at, and information on what data the patch will add to the font.</p>
</ol>
<p class="note" role="note"><span class="marker">Note:</span> this is a highly simplified description of creating an incremental font, a more in-depth discussion of generating an encoding and
requirements on the encoding can be found in the <a href="#encoding">§ 7 Encoding</a> section.</p>
<h3 class="heading settled" data-level="1.4" id="performance-considerations"><span class="secno">1.4. </span><span class="content">Performance Considerations and the use of Incremental Font Transfer</span><a class="self-link" href="#performance-considerations"></a></h3>
<p>Using incremental transfer may not always be beneficial, depending on the characteristics of the font,
the network, and the content being rendered. This section provides non-normative guidance to help decide
when incremental transfer should be utilized.</p>
<p>It is common for an incremental font to trigger the loading of multiple patches in parallel. So to maximize performance, when
serving an incremental font it is recommended that an HTTP server which is capable of multiplexing (such as <a data-link-type="biblio" href="#biblio-rfc9113" title="HTTP/2">[rfc9113]</a> or <a data-link-type="biblio" href="#biblio-rfc9114" title="HTTP/3">[rfc9114]</a>)
is used.</p>
<p>Incrementally loading a font has a fundamental performance trade off versus loading the whole font.
Simplistically, under incremental transfer less bytes may be transferred at the potential cost of
increasing the total number of network requests being made, and/or increased request processing
latency. In general incremental font transfer will be beneficial where the reduction in latency from
sending less bytes outweighs additional latency introduced by the incremental transfer method.</p>
<p>The first factor to consider is the language of the content being rendered. The evaluation report
contains the results of simulating incremental font transfer across three categories of languages
(<a href="https://www.w3.org/TR/PFE-evaluation/#langtype">Progressive Font Enrichment: Evaluation Report § langtype</a>). See it’s conclusions <a href="https://www.w3.org/TR/PFE-evaluation/#conclusions">Progressive Font Enrichment: Evaluation Report § conclusions</a> for a discussion of the
anticipated performance of incremental font transfer across the language categories.</p>
<p>Next, how much of the font is expected to be needed? If it’s expected that most of the font will be
needed to render the content, then incremental font transfer is unlikely to be beneficial. In many cases
however only part of a font is expected to be needed. For example:</p>
<ul>
<li data-md>
<p>If the font contains support for several languages but a user is expected to only render content
in a subset of those languages.</p>
<li data-md>
<p>If the content being rendered uses a small subset of the total characters in a font. This is
often the case for Chinese, Japanese, Korean, Emoji, and Icon fonts.</p>
<li data-md>
<p>Only a small amount of text is being rendered. For example a font that is only used for a
headline.</p>
</ul>
<p>An alternative to incremental transfer is to break a font into distinct subsets (typically by script)
and use the unicode range feature of @font-face to load only the subsets needed. However, this can alter
the rendering of some content <a href="https://www.w3.org/TR/PFE-evaluation/#fail-subset">Progressive Font Enrichment: Evaluation Report § fail-subset</a> if there are layout rules between characters in
different subsets. Incremental font transfer does not suffer from this issue as it can encompass the
original font and all of it’s layout rules.</p>
<h4 class="heading settled" data-level="1.4.1" id="reduce-requests"><span class="secno">1.4.1. </span><span class="content">Reducing the Number of Network Requests</span><a class="self-link" href="#reduce-requests"></a></h4>
<p>As discussed in the previous section the most basic implementation of incremental font transfer will
tend to increase the total number of requests made vs traditional font loading. Since each augmentation
will typically require at least one round trip time, performance can be negatively impacted if too many requests
are made. Depending on which patch types are available and how much information is provided in the <a href="#font-format-extensions">patch mapping</a>, a client might preemptively request patches for code points that are not
currently needed, but expected to be needed in the future. Intelligent use of this feature by an
implementation can help reduce the total number of requests being made. The evaluation report explored
this by testing the performance of a basic character frequency based <a href="https://www.w3.org/TR/PFE-evaluation/#codepredict">code point prediction</a> scheme and found it improved overall performance.</p>
<h2 class="heading settled" data-level="2" id="opt-in"><span class="secno">2. </span><span class="content">Opt-In Mechanism</span><a class="self-link" href="#opt-in"></a></h2>
<p>Web pages can choose to opt-in to incremental transfer for a font via the use of a CSS font tech
keyword (<a href="https://drafts.csswg.org/css-fonts-4/#font-tech-definitions"><cite>CSS Fonts 4</cite> § 11.1 Font tech</a>) inside the ''@font-face'' block. The keyword <code>incremental</code> is used to indicate the referenced font contains IFT data and should
only be loaded by a user agent which supports incremental font transfer.</p>